The Effect of Weather on the Winter Activity of Old-Field Rodents

Abstract

As a part of basic studies of ecological change on the AEC Savannah River Plant Area special attention has been given to the old field ecosystem by a faculty-student team of the University of Georgia. Seasonal and annual changes in the “standing crops” of major plant and animal groups have been followed. For sampling small mammals, extensive use has been made of a standard trapline technic originally developed by Calhoun (1948). The basic trapline unit consists of 20 stations with three traps per station set for three nights. We placed stations 25 feet apart so that small as well as large fields could be sampled (for larger fields more than one unit is often used). Peromyscus polionotus has been, and still is, the most important small mammal in former agricultural fields now abandoned for four growing seasons. Mus musculus, Reithrodontontomys humulis and Cryptotis parva occur in smaller numbers with an occasional Sigmodon hispidus straying into these fields from its normal habitat of older successional stages. Armed with home range data obtained from live-trapping in 1952–54 by Leslie B. Davenport, another member of our team, we have been hopeful of converting numbers per trapline into at least reasonable estimates of animals per acre. Calhoun, Casby and Brant (1955) have recently shown that “space-relative” density can be calculated from ratios between catches of the first two nights, provided the home range is known and the catch on different nights is not greatly affected by the weather. Since Peromyscus in these southern regions reaches a seasonal peak in abundance in winter or early spring (see also McCarley, 1954), sampling should be done then if peak populations of different areas are to be compared (not in late summer or fall as would be the case with many species in the northern United States). Even though winters are relatively mild in the region under study, the question of weather effects becomes a critical one in any attempt to calculate abundance from trapline catch. Accordingly, between January 6 and March 20 a total of 53 traplines were run in a special study of the effect of weather on catch.